In order to get consistent behaviour on linux with various gcc versions,
I am defining _GNU_SOURCE in ompi_config.h and changing the definition
of asprintf, etc.
Problems:
1) autoconf was defining _GNU_SOURCE for its tests, but we were not
setting it
2) g++ sets it by default (I think)
3) some version of gcc set it by default
If somebody knows a consistent solution for these problems then we can
revert to not setting it.
This commit was SVN r2230.
5ce8144d91